DC Motor overvoltage

I want to run a 3.6V DC motor from a 12V battery. The motor will not run continously, only 0.5sec at a time with several seconds delay before it will run again.

Can I do this without damaging the motor?

(I'd like to avoid using a regulator or PWM if possible, also the extra torque from higher voltage would be useful.)

If you know the motor current at normal operation (you can measure this ;-) ), just add a series resistor. Lessee, I think R = E/I,

so your resistor would be (12 - 3.6)/(Imotor) ohms.

Good Luck! Rich

However, what is important is the current at startup in this operating mode. That will be higher than at normal load. In addition, the resistor while cheap will not help at no load or light load- (speed will be high and the speed will change more rapidly under load. Also- what kind of motor? shunt, permanent magnet, series? it makes a difference.

The motor draws ~3amp, so I will need a ~30W resistor, which means either a bulky wirewound resistor or an expensive film resistor. Since it won't run continously I can probably use a smaller resistor, but I don't know how small.

mode.

I'm guessing series, because this motor comes from en electric screw driver, but I'm not 100% sure.

I'm going to use the motor as an actuator for a ball valve, obviously with significant gear reduction. I will leave the motor on for

My guess is that you stand a good chance of doing some kind of mechanical damage to the motor, particularly in light of the fact you will be inpacting it on a stop With 3 times the voltage, the starting torque may be more than the motor can handle.

If you are going to use a screwdriver motor -why not keep the gears that go with it? If it is a series motor, you can limit current with a resistor. I would suggest that as you are running at 3 times the voltage, use 2 times the measured locked rotor resistance in series- rate the resistor on the basis of the current under locked rotor conditions. This will limit the current to what the motor normally takes in this condition and stall torque to what it is normally. The speed will drop more under load than it would normally. A screwdriver motor is expected to stall so you could reduce the extra resistance somewhat. Watch for arcing or sparking at the brushes- I think the windings will take the overcurrent for the time under consideration but the brushes may be a problem.

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I haven't tried it, but I would have thought the answer was No. And if it wasn't for your final proviso, I'd have suggested a simple 12V PWM approach. Apart from probably minimising risk of motor damage, that would allow you to secure extra torque.

FWIW, that's what I was considering doing for my curtain controller (subject of recent threads). That too uses a screwdriver motor and gearbox. Mine was originally powered by 2.4 V (2 x 12 V Nicads). But that didn't quite cut the mustard. So I increased it to 3.6 V (with a permanent trickle charge sourced from an existing convenient 12 V DC supply). In initial tests that seemed to give too violent an action (especially dramatic when the curtains opened automatically at dawn ). But it turned out that the necessary longish wiring to the motor provided sufficient resistance to tame it.

BTW, I originally tried my 2.4 V motor at progressively higher voltages from my bench power supply, but quit at 6 V.

Did you also consider a limiting microswitch? I'd be interested in what stall-detection circuitry you are using.